Molecular and Crystal Structure of 7-Fold V-Amylose Complexed with 2-Propanol

Single crystals of amylose V2-propanol, i.e., amylose cocrystallized with water and 2-propanol, were prepared from synthetic linear amylose. When observed under frozen hydrated conditions, these crystals yielded base-plane electron diffraction diagrams containing more than 100 independent diffraction spots, whose intensities were used to solve the crystal structure of this complex. The molecular and crystal structure of amylose V2-propanol clearly indicated that the amylose molecules were organized in 7-fold left-handed helices, with 2-propanol and water molecules located as guests both within and between the helices. The V2-propanol unit cell contains four helices, distributed in two antiparallel pairs of parallel helices organized along the P212121 symmetry. The helices are organized along alternating motifs of four helices in a close-packed hexagonal arrangement together with four others in a nearly square organization surrounding a central column of water and 2-propanol. Whereas the location of the amylose helices is well established in the unit cell, the coordinates of the guest molecules could not be defined with certainty, most likely due to a positional disorder. A tentative model of the guest molecule distribution is presented, which consists of two 2-propanol and two water molecules within the helical cavity together with four water molecules and two 2-propanol molecules between the helices. The mobility of the guest and its description as a continuum, rather than at fixed crystallographic positions, explain why so many structures isomorphous to V2-propanol can be obtained with different guest molecules, while yielding similar electron diffraction diagrams.